Developments in combustion engine technology have shown that compression ignition engines, frequently referred to as diesel-cycle engines, can be fuelled with gaseous fuels instead of diesel without sacrifices in performance or efficiency. Examples of such gaseous fuels include natural gas, methane, propane, ethane, gaseous combustible hydrocarbon derivatives and hydrogen. Substituting diesel with such gaseous fuels generally results in cost, availability and emissions benefits over diesel. Examples of emissions benefits include, compared to conventional diesel-fuelled engines, a reduction of at least about 75% in the level of particulate matter (commonly known as soot), a reduction by about half in the level of oxides of nitrogen (commonly known as NOx), and a reduction by about 25% in the level of carbon dioxide.
However, a problem with gaseous fuels such as natural gas is that, compared to diesel fuel, much higher temperatures and pressures are needed for auto-ignition. To overcome this problem an ignition assist mechanism can be employed to control ignition of gaseous fuels. One such ignition assist mechanism, which allows the major components and operational characteristics of diesel engines to be preserved, involves the injection of a small amount of more auto-ignitable pilot fuel such as conventional diesel fuel, to control the ignition of the gaseous fuel.
When diesel fuel is used as a pilot fuel the quantity of diesel fuel that is consumed can be less than 5% of the total fuel consumed on an energy basis. Delivering such a small amount of diesel fuel to an engine has consequences that do not normally occur in a conventional diesel engine, when a larger amount of diesel fuel is delivered to the engine. For example, in a conventional diesel engine, with the larger diesel mass flow rates being injected into the combustion chamber, and because the temperature of the diesel fuel delivered to the fuel injection valves is much lower than the temperature of the cylinder head and the temperature within the combustion chamber, the diesel fuel itself acts as a coolant so that the temperature of the diesel fuel within a conventional injection valve does not normally rise above the initial boiling temperature of liquid diesel fuels. It is easier to inject more fuel through the injection valve when the diesel fuel is in a liquid state, so ideally the fuel remains a liquid until it exits the nozzle orifice and is vaporized inside the combustion chamber.
Diesel fuel is not composed of a simple compound, meaning that diesel fuel is made up of a mixture of different compounds each one having a different boiling point. There are different grades of diesel fuel with different compositions, but, in general, diesel fuels boil between an initial boiling point and a final boiling point. The initial boiling point is the lower end of the temperature range and it is at this temperature that some of the lighter compounds are vaporized. That is, when the diesel fuel temperature is below the initial boiling point, all of the diesel compounds will be in a liquid state. The final boiling point is the higher end of the temperature range and above this temperature all of the diesel compounds can be vaporized. When the temperature of the diesel fuel is between the initial and final boiling points, the fuel can be in two phases. For example, a common grade of diesel fuel is known as “No. 2 diesel fuel” and at atmospheric pressure this fuel has an initial boiling point of about 125° C. and a final boiling point of about 400° C. A distillation profile for No. 2 diesel fuel under atmospheric pressure is depicted in FIG. 7, with the boiling point for the different compounds plotted against percent (by weight). For example, with reference to this profile, a temperature of 350° C. is higher than the boiling point of over 90 percent of the compounds. In this example, if No. 2 diesel fuel is heated to 350° C., 90 percent of the fuel would be vaporized and only 10 percent of the fuel would remain in liquid form. At higher pressures, the profile shifts to higher temperatures, because the compounds will remain in a liquid state at higher temperatures when it is at higher pressure. Accordingly, the initial and final boiling points have values that change according to the current pressure of the pilot fuel.
Pilot fuel injection pressure can be higher than 20 MPa so all of the pilot fuel compounds held within the fuel injection valve can be superheated during engine operation and can remain in a liquid state at temperatures higher than 125° C. During engine operation the pilot fuel pressure within an injection valve is held at a substantially constant high pressure upstream from the valve seat so partial vaporization within the injection valve is not normally a problem. However, because the pilot fuel pressure fluctuates further below injection pressure, for example, downstream from the valve seat during an injection event, to prevent partial vaporization of the pilot fuel inside the nozzle orifices, it is desirable to keep the pilot fuel temperature inside the pilot fuel injection valve below the lower initial boiling point associated with the pilot fuel when it is downstream from the valve seat. With the low mass flow rate associated with pilot fuel or other super low flow applications, there can be a problem with keeping the pilot fuel temperature below the lower initial boiling points associated with lower pressures.
Partial vaporization of the fuel is undesirable because the heavier compounds which are harder to vaporize tend to be compounds that can form a sticky tar-like residue when the lighter compounds are vaporized. Accordingly, it is undesirable for the pilot fuel within the injection valve to be held at a temperature that will result in the pilot fuel being injected at a temperature that is between the initial and final boiling points for the pilot fuel when it is at a lower pressure downstream from the valve seat. At temperatures above 300° C. in some injection valves diesel fuel is known to start yielding precipitates including wax, varnish, and sulfur and under higher temperatures diesel fuel can turn to “coke” a solid residue of impure carbon that forms after the removal of volatile hydrocarbons by distillation.
Therefore, if a pilot fuel injection valve is operated with a pilot fuel temperature that is too high, a tar-like liquid mixture can form that is more likely to adhere to the valve surfaces downstream from the valve seat where the pilot fuel pressure is lower (and the initial boiling point is lower), and this can result in deposits which can eventually restrict fuel flow through the injection valve and/or interfere with valve operation. Even in areas where the diesel fuel velocity can be high, such as in the nozzle orifices, when the diesel fuel is partially vaporized tar-like compounds can be viscous enough to stick to the orifice walls.
When diesel fuel is employed as a pilot fuel, because the mass flow rate is much lower than when diesel is used as the main fuel, this can result in the temperature of the diesel fuel rising above the initial boiling point. Therefore, unlike conventional fuel injection valves, because of the lower mass flow rate for a pilot fuel versus a main fuel, with pilot fuel injection valves there can be a problem with keeping the temperature of the diesel fuel below the initial boiling point.
Another problem with gaseous-fuelled engines that employ a liquid pilot fuel is that combustion chamber deposits can collect on the surfaces of the fuel injection valve's nozzle that are exposed to the combustion chamber. For example, such deposits can form on the nozzle near the injection orifices and in the sac area of the gaseous and pilot fuel injection valves. Such deposits can form a layer that can grow in thickness and eventually interfere with the fuel flow through the orifices of the respective gaseous and pilot fuel injection valves. In some cases the nozzle orifices can be obscured to the degree that fuel flow into the combustion chamber is reduced, resulting in a drop in engine performance.
Pilot fuel and engine oil can both be sources of combustion chamber deposits. However improvements to modern engines have reduced engine oil consumption so that pilot fuel is believed to be the major contributor for combustion chamber deposits. Combustion chamber deposits can occur on both the gaseous and pilot fuel injection valves.
In a conventional direct injection engine, typically there is a heat transfer path from the injection valve nozzles, through the associated injection valve bodies, and to a liquid cooled cylinder head, within which the injection valves are mounted. This heat transfer path takes heat away from injection valve nozzles. In such an arrangement, the temperature of an injection valve nozzle can be between around 140° C. and 275° C. even when the peak temperature in the combustion chamber is at least 700° C. and the average temperature therein is at least about 425° C. This would not be a significant problem in an engine that is fuelled solely with gaseous fuel. However, when a liquid pilot fuel is used as the ignition assist mechanism, the relatively cool surface of the injection valve nozzle can cause the pilot fuel to condense thereon, if the surface temperature is lower than the final boiling point of the diesel fuel compounds. For a nozzle surface with a temperature that is between the initial and final boiling points, because the heavier tar-like compounds have higher boiling points, the condensate that will form on the exposed nozzle surfaces will comprises more of these sticky tar-like compounds. Over time, these deposits will decompose, losing hydrogen atoms and forming a hard carbon layer.
Accordingly, there is a need for a fuel injection system for a gaseous fueled engine with liquid pilot fuel ignition that avoids the problems that can arise if the heavier compounds of diesel fuel are deposited inside the pilot fuel injection valve or on the surfaces of the gaseous fuel or pilot fuel injection nozzles that can come into contact with the pilot fuel that is injected into the combustion chamber.